The present invention relates to a computer terminal which combines features of graphics terminals with those of alphanumeric terminals. More particularly, the present invention relates to a bit-mapped computer terminal capable of operating in a graphics mode or in an alphanumeric mode.
Modern display terminals are packaged and sold as sophisticated equipment that is far more complex than a simple cathode ray tube (CRT) display unit. These terminals are frequently sold together with a keyboard, a data link for receiving data from a computer, and microprocessors for carrying out the functions of driving the CRT, monitoring the keyboard, and controlling communications through the data link. The microprocessors also control certain features such as scrolling and cursor controls and other controls that facilitate use of the terminal. These microprocessors also allow the terminal to perform limited data processing or word processing functions, and include special modules containing read-only memories. These modules may be in the form of ROMs, PROMs, EPROMs, or other suitable memory, and may be used to store the software instructions for operating the microprocessor. Displayed character fonts and the like may be determined by the information stored in the read-only memory.
The sophistication of modern alphanumeric terminals provides greater control over displayed information. For example, various character attributes, such as whether a particular character or group of characters blinks, is displayed in reverse-video (i.e. black character on a white background), or is highlighted, can be stored in terminal memory. This added flexibility enhances the display and permits tailoring of the display to the particular desires of the user.
A typical display device provides a plurality of horizontal scans per screen. For example, a single display screen might include approximately 400 horizontal scans per screen. The horizontal scanning frequency may be approximately 32 KHz and the vertical scanning frequency, i.e. the frequency at which the entire display is updated may be approximately 60 Hz. A vertical scanning frequency of 60 Hz permits the terminal to synchronize the vertical retrace with the standard AC line frequency. Of course, when a terminal is marketed throughout the world, the standard AC line frequency may vary from place to place. An AC line frequency different than 60 Hz, however, will seriously affect the operation of a terminal intended for use with a 60 Hz power supply.
Terminals may operate as either an interlaced display or a non-interlaced display. A non-interlaced display updates the entire display during each vertical trace. In other words, each of the raster lines is updated for each vertical trace. An interlaced display, on the other hand, only updates every other raster line during each vertical trace. For example, raster lines 1, 3, 5 . . . , 399 would be updated during a first vertical trace and raster lines 2, 4, 6 . . . , 400 would be updated during a second vertical trace. Raster lines 1, 3, 5 . . . , 399 would again be updated in a third vertical trace.
An interlaced display can be used to increase the vertical resolution of a display. During a first vertical trace, each of the 400 raster lines could be updated. A second vertical trace could then be vertically offset so that the first raster line of the second vertical trace falls between the first and second raster lines of the first vertical trace. In this way, the resolution of a 400 scans per screen display can be increased to 800 scans per screen. Of course, the amount of time required to fully update a screen is doubled.
U.S. Pat. No. 4,482,919 to Alston et al discloses an apparatus for providing a multilaced raster scan system wherein interlacing is provided by selectively varying a DC bias signal which controls the position of the raster lines on the display. Referring to FIG. 5 of the Alston et al patent, a typical interlaced display is illustrated wherein the raster lines of a second field are vertically offset from the raster lines of a first field. Other patents which relate to interlaced displays include U.S. Pat. No. 4,307,421 to Smit, U.S. Pat. No. 4,598,236 to Hepworth, and U.S. Pat. No. 4,608,602 to Grantham-Hill. Hill.
Terminals also may use a bit-map scheme to provide graphics capability. In a standard bit-mapped monochrome terminal the pixels (picture elements) on a display monitor are in one-to-one correspondence with bit locations in a memory. For example, if the bit location corresponding to a particular pixel contains a logic "1", then the pixel will be lighted. If the memory bit location contains a logic "0", then the pixel remains dark. If a terminal has 1056 pixels per raster line and 400 raster lines per screen, then the bit-map memory will require at least 53 Kbytes of memory. Due to additional information required for bit-mapped color terminals, these terminals require several bits for each pixel in the display. Of course, if the memory is not used with the utmost efficiency, the memory requirements of a bit-mapped terminal will increase.
Bit-mapped terminals require a large amount of memory relative to a standard alphanumeric display terminal. In order to provide the character attribute flexibility of an alphanumeric terminal in a bit-mapped system, an exorbitant amount of memory would be required. Each pixel of the display would require an additional bit in memory for each character attribute. Thus, in a bit-mapped terminal which would normally require 64 Kbytes of memory, an additional 256 Kbytes of memory would be required to permit the selection of four bit-mapped character attributes.
In everyday use, it would be convenient to have a terminal which is capable of easily operating as a graphics terminal and as a standard alphanumeric terminal. For example, an application program might use graphics to guide the user in running the application software. Figures could be used to represent the operation of the program. Once the operation of the program has been illustrated, the terminal would be used as a standard alphanumeric terminal. Preferably the terminal would operate as a high-resolution interlaced display when acting as a graphics terminal. In normal use as an alphanumeric terminal, it would often be unnecessary to provide a high-resolution display. Of course, it is desirable to maintain the costs of such a terminal as low as possible.